1. Field of the Invention
This invention relates generally to gas flow meters, and more particularly to a method and apparatus for dispensing and measuring natural gas into vehicle tanks.
2. State of the Prior Art
Dispensing gaseous fuels, such as compressed natural gas, into motor vehicle tanks at the consumer retail point of sale presents problems that are either not encountered or not significant in the more conventional liquid fuel distribution and retail sales apparatus and systems. For example, natural gas is transferred and stored under high pressure, which, according to the ideal gas law, varies as a function of temperature, and which requires special fittings, tanks, and safety considerations that are more complex than liquid fuel handling equipment and components. Also, measuring the quantity of a gas being dispensed at the point of sale into a customer's motor vehicle tank is not as easy as measuring a liquid fuel.
Tanks for storing compressed natural gas aboard vehicles are manufactured to withstand pressures within normal ranges, such as 3,000 to 4,000 p.s.i.g., with an extra safety margin. Because gas pressure varies as a function of temperature, and because the temperature of a fuel tank on a motor vehicle cannot be controlled easily or reliably the tank manufacturers rate their tanks for a maximum safe operating pressure at a convenient and normal "standard temperature", usually 70.degree. F. (19.degree. C.). For example, a tank rated at 3,000 p.s.i.g. at 70.degree. F. (19.degree. C.) is designed to withstand safely whatever pressure that amount of gas will exert at whatever higher temperatures it would likely encounter in extreme environments. According to the ideal gas law, PV=nRT, where the volume V of the tank, the moles n of the gas, and the universal gas constant R, are all constant for a given tank containing a given amount of gas, the pressure P varies in a direct, one-to-one relation to the absolute temperature T. Therefore, a tank filled to the maximum rated pressure of 3,000 p.s.i.g. at the standard temperature of 70.degree. F. (19.degree. C.) will have a pressure of about 3,455 p.s.i.g. at 150.degree. F. (63.3.degree. C.). Consequently, the tank is built with a margin of safety to withstand such higher pressures at reasonably foreseeable higher temperatures. However, it would be unsafe to fill a tank at the standard 70.degree. F. (19.degree. C.) to a pressure higher than its maximum rated 3,000 p.s.i.g. pressure, because, if the tank is then exposed to a higher extreme but foreseeable temperature, the resulting higher pressure could exceed the safety margin and possibly rupture the tank. For the same reason, filling the tank to the rated 3,000 p.s.i.g. pressure at a lower temperature, such as 32.degree. F. (0.degree. C.) would result in a potentially dangerous overfill, if the vehicle is exposed to a substantially higher temperature shortly thereafter, which could increase the pressure to unsafe levels.
At the same time, it is inefficient and undesirable to underfill a customer's vehicle tank, because underfilling reduces driving range for the customer and prematurely aborts a sale for a vendor. Consequently, it is desirable to provide a complete, but proper fill to a maximum pressure that is compensated for the then current temperature in a manner that would result in the proper 3,000 p.s.i.g. maximum pressure if the actual temperature was raised to the standard 70.degree. F. (19.degree. C.) rating temperature.
Of course, it is also desirable to get a proper measure of the gas delivered to a customer's vehicle tank as a basis for calculating the point of sale price to be charged to the customer or for other record purposes.
A significant problem, both for determining the maximum tank fill and for measuring the amount of fuel delivered in a gas rather than a liquid setting is that because of its compressibility, the quantity of gas is difficult to measure independent of its temperature and pressure, which are usually different in the customer's vehicle tank than in the vendor's dispenser. However, it is preferred, at least from the vendor's standpoint, if not also for purposes of governmental regulation of weights and measures, to not have to rely on instruments or transducers in the customer's tank to measure pressure and temperature.
Our U.S. Pat. No. 5,238,030, which is incorporated herein by reference, was directed to a method and apparatus for determining a temperature compensated maximum tank pressure and automatic fill cut-off when such maximum pressure is reached and for measuring the mass of the natural gas fuel delivered, all from instruments or transducers in the dispenser, rather than in a customer's tank. However, restrictions and pressure losses through the delivery hose, tank valves, and lines made accurate cut off pressure still difficult to measure with that invention. Therefore, we developed the invention of our U.S. Pat. No. 5,259,424, which is also incorporated herein by reference, to provide an improved and more accurate method and apparatus for measuring the size of the customer's tank and then determining the additional mass of gas needed to get a complete fill of natural gas in the customer's tank corrected for standard pressure and temperature, all for use in combination with a mass flow measuring method and apparatus for determining maximum fill cut off and the amount (mass) of the gas actually dispensed.
While the sonic nozzle flow meter system used in our U.S. Pat. Nos. 5,238,030 and 5,259,424 described above is reasonably accurate and probably more accurate than most other flow meters used in natural gas dispensers, it assumes a constant molecular composition gas mass at (standard) pressure and temperature. Therefore, it loses accuracy when the molecular composition of the natural gas varies. It also maintains accuracy only in a restricted flow range, loses accuracy in subsonic flow, and is a function of back pressure. Therefore, there is still a need for more improvement in metering accuracy for measuring the amount of gas dispensed into a vehicle tank without having to rely on pressure and temperature transducers in the vehicle tank itself.